The present invention relates to signal processing methods and apparatus for quadrature modulated color television systems and signals. More particularly, the present invention relates to the suppression of unwanted cross color artifacts incident to diagonal luminance transitions in a color television signal.
In quadrature modulated color television systems, the color information is modulated in quadrature upon a subcarrier having a frequency related to the line scan frequency in a manner which interleaves the color information about the subcarrier between energy spectra of the luminance base band signal. When the color information and the luminance information overlap, due to imperfect encoding techniques as is commonly the case, the color demodulator demodulates the overlapping high frequency luminance as a rainbow pattern which constitutes an unwanted artifact in the picture. This rainbow pattern is commonly referred to in the art as cross-color.
An article entitled "A Simplified Cross-Color Suppression Circuit for Color Television Receivers" by Sasaki, Furuhashi and Nagaoka, in IEEE Transactions on Broadcasting and Television Receivers, August 1978, pp. 178-183 proposed an arrangement for eliminating all chrominance information at the vicinity of a high frequency luminance transition in order to avoid the cross-color problem. These workers at the Wireless Research Laboratory of Matsushita Electric Industrial Co., Ltd., Osaka, Japan, proposed measuring the luminance spectrum at approximately 2 MHz via a filter which had no passband response at the higher color subcarrier frequency (3.58 MHz). A unidirectional pulse was developed from each picture transition, and the pulse was used to switch off the chrominance information at each detected transition, irrespective of the angle of the transition in the picture. While the solution to cross-color artifacts proposed by these workers was satisfactory for low cost television receivers, it is a brute force, crude approach, and whenever a luminance transition occurs which is unaccompanied by a concomitant chroma transition, the action of killing the chroma at the transition leaves a distinctly perceptible monochrome line at the transition, i.e., another unwanted picture artifact.
With the advent and adoption of low cost comb filters enabling recursive signal processing (typically at the line scan frequency) of the television signal at the color decoder-receiver to separate luminance and chrominance components of the signal, some but not all of the undesired cross-color artifacts were removed inherently in the comb filter processing. For example, in the instance of a vertical line appearing in the horizontally scanned picture, the summation of adjacent lines in order to separate chroma from luminance eliminates all luminance from the chroma by phase cancellation inherent in the summation process. Thus, it would be most undesirable to suppress chroma at the vicinity of a vertical line transition in a comb filter system, since no cross color is present, and since the chroma suppression adds a new, monochrome artifact to an otherwise undistorted picture signal.
Horizontal line transitions occur at the picture field rate e.g. 60 Hz. These very low frequency transitions are unaccompanied by cross-color rainbow patterns, and luminance transitions at the field rate are easily overlooked by the band pass filtering technique proposed by the Matsushita workers in the article referred to above.
While ideal comb filters perfectly separate chroma and luminance recursively line by line, and inherently eliminate in-line (vertical line) cross-color rainbow patterns, these filters do not work for transitions which are diagonal. By diagonal is meant any line transition in the picture content which is more than approximately twenty degrees from the vertical. The failure of comb filters to eliminate cross-color patterns in such picture conditions is better understood by realizing that such transitions do not occur at the same line scan time from line to line, and therefore they will not be cancelled when a present line of video is subtracted from a one line delayed line of video.
Not only do comb filters not operate effectively for cross-color elimination at diagonal luminance transitions in the picture content, they tend to introduce additional unwanted artifacts into the picture, particularly when multiple line delays are incorporated into the system. For example, if three adjacent lines are being recursively processed, the diagonal transition causes an average of three lines of unwanted crosscolor artifacts to be put out by the apparatus.
A hitherto unsolved need has arisen for a control method and apparatus for eliminating cross-color patterns from a quadrature modulated color television signal undergoing recursive processing with delay lines and summing nodes (comb filters).